This invention has as an object provide an improved device for generating acoustic shear waves in the earth, particularly useful in the field of seismic prospecting.
In seismic prospecting, the location of the reflecting layers of the subsoil is often determined by transmitting through the earth longitudinal acoustic waves (P waves) which propagate according to a compression mode. But there is also used transverse or shear waves (S waves) which propagate according to a distortion or shear mode. The shear waves have interesting applications since their wave length is, at equal frequency, lower than that of the longitudinal waves and they have a better resolving power. In addition, some of them (SH waves) have the particularity, in certain conditions, of being not subjected to any conversion in the presence of stratigraphic discontinuities and the recordings obtained from said waves are simpler.
A known device, adapted to generate acoustic shear waves in the earth, is described, for example in the French Pat. No. 2 276 599. It essentially comprises a target member provided with a surface coupled to the earth and at least one striking lateral surface substantially perpendicular to that coupled with the earth, a mass for striking the target member, a rigid arm pivotable with respect to a platform for supporting the mass and guiding its fall along a circular trajectory towards the lateral surface, and operating means for lifting the mass, by pivotation of the rigid arm with respect to the platform, from a lower position, where the mass is in contact with the lateral striking surface, to an upper position, wherefrom the mass falls. The target-member is connected through deformable coupling means to the lower part of a supporting rigid structure displaceable with respect to the frame of the carrying vehicle between a lower position where the target-member is in contact with the earth to an upper position for transportation.
The target-member being laid down onto the earth, the mass, previously lifted by pivotation of the arm, is released and strikes the lateral surface of the target-member. The impact generates acoustic shear waves which are transmitted to the earth. In order to improve the efficiency of the device, the coupling of the target-member with the earth-surface is enhanced by application of a vertical force thereto. For this purpose, the target-member comprises, at its upper part, roller paths whose spacing is adapted to that of the vehicle axles. The target-member which has been transported to the selected triggering location, is laid down on the earth, then the vehicle is moved so that its rear wheels bear on the roller paths. Once the seismic "shots" have been effected at the selected location, the vehicle is moved again so that its rear wheels release the target-member and comes back to the position required for securing again the rigid structure of the support to the target-member and hoists up the latter to its transportation position. It results from the used coupling mode that the repetition frequency of the successive triggerings of the device at separate locations is essentially limited by the time required for moving the target-member.